[49.05] Using 2MASS to Detect Companions of Blue Stars.

M. D. Reed (Iowa State University)

Subdwarf B (sdB) stars are extreme horizontal branch (EHB)
stars that are thought to consist of a 0.5M\odot helium
burning core with less than 10-4M\odot of hydrogen
in the envelope. As such, they are extremely blue with
effective temperatures between 25,000 and 40,000 kelvin. The
origin of these stars is poorly understood as they require
sufficient mass to undergo a degenerate helium flash as well
as an enhanced mass loss mechanism to account for the thin
hydrogen layer. One solution is to have the EHB star in a
binary such that Roche lobe overflow occurs near the tip of
the red giant branch (Green, Liebert, & Saffer, 2000, AAS,
197.4601).

To test this theory, we need to determine how often sdB
stars occur in binaries. One relatively simple way to do
this is to search for cooler companions. Subdwarf B stars
are relatively small, with radii of approximately 0.15-0.3
R\odot, such that they do not outshine cooler
companions at longer wavelengths. We have used the 2MASS
survey to obtain the J, H, and Ks magnitudes for
\emph{every} star classifed as sdB or sdOB from the Palomar
Green (Green, Schmidt, & Liebert, 1984, ApJ, 287, 320,
Edinburgh-Cape (Kilkenny et al. 1998, MNRAS, 296, 320), and
Hamburg-Schmidt (Heber, 2001, private communication) surveys
available in the 2MASS Second Incremental Release Point
Source Catalog. We then compare colors with those obtained
from models (Kurucz, 1996, IAUS, 176, 523).

The author(s) of this abstract have provided an email address
for comments about the abstract:
mreed@iastate.edu